This invention relates to resistance elements, and more particularly to bridge elements for electro-explosive devices.
Electro-explosive devices (EED's) such as primers, detonators, and squibs, are well known in the art. These devices generally include a pair of lead wires which are connected through a bridge wire or other bridge element that is in contact with a deflagrating charge. The device generally has a metallic case. The bridge element is a resistance element that is usually in the form of a wire of circular cross section. The bridge wire usually has a resistance which is appreciably greater than the resistance of the lead wires. Passage of an electric current through the leads and the bridge element causes the latter to be heated, thereby firing the deflagrating charge.
A well known safety hazard of some electro-explosive devices is that they can be accidentally fired by static electricity. A safety requirement of relatively recent origin for EED's in some military specifications is the "one ampere-one watt no fire" requirement. This requirement states that a device must be capable of dissipating one watt of power while one ampere of current is passed through the bridge element without firing. This indirectly fixes the desired combined resistance of the lead wires and the bridge element at one ohm. The combined lead wire and bridge element resistance is generally held between 0.9 and 1.1 ohms. If the total resistance is too low, the one watt requirement will not be met; if it is too high, excessive heating due to higher power dissipation will result.
The passage of current through the bridge element causes its temperature to increase considerably. Heat is transferred from the bridge element to the deflagrating charge which surrounds the bridge. The charge conducts the heat to the body of the device where it is dissipated. To increase the rate of heat transfer and thereby minimize the bridge element temperature at any given current level, a bridge element shape having a greater ratio of external surface area to cross sectional area than the conventional round wire shape is required. For this reason foil bridges have been used in place of bridge wires in devices meeting the one ampere-one watt no fire requirement.
A foil bridge element which is presently in use is illustrated in FIG. 1. This device is generally circular in shape with sawtooth edges, and includes a narrow resistor section 11 of high resistance per unit length which runs along a diameter of the circle and which connects two much wider portions 12 and 13 which have sawtoothed outer edges that lie along the circumference of a circle. The outer portions 12 and 13 have holes 14 for alignment of the bridge element with respect to the lead wires (not shown) to which the bridge element is connected. Since the portions 12 and 13 are much wider than the resistor section 11, nearly all of the resistance of the bridge element is in the resistance section 11. As a corollary of this, the bridge element shown in FIG. 1 would have an essentially constant resistance regardless of the points in portions 12 and 13 at which the lead wires are connected.
The foil bridge element shown in FIG. 1 has a typical thickness of about 0.001 inch. A desirable composition for this element is an alloy containing 20% chromium, 2.75% aluminum, 2.75% copper, all percentages by weight, balance nickel. The resistivity of this alloy is 134 microhm-centimeters, or 800 ohms per circular mil foot. Thus the resistance of the linear portion of the bridge element in FIG. 1 would be about one ohm when the diameter of the circle is 0.17 inch and the width and thickness of the linear portion 11 are 0.01 and 0.001 respectively.
Processes for making foil resistance elements such as the above are known in the art. One such process is described in T. D. Schalbach and D. K. Rider, "Printed and Integrated Circuitry Materials and Processes", McGraw-Hill, New York 1963, pages 83-87.
A disadvantage of presently known bridge elements including the bridge element shown in FIG. 1, is that they have a fixed resistance. Since lead wires of various EED designs may vary in either length diameter or material, and consequently may vary in resistance, it is impossible to achieve the desired one ohm of combined lead wire and bridge element resistance with one bridge element design.